A Passive Optical Network (PON) comprises an Optical Line Termination (OLT), which resides in a Central Office (CO) and further comprises user modems, called Optical Network Terminals (ONT) or network units, called Optical Network Units (ONU). The OLT services a number of ONT's or ONT's, typically connected in a tree arrangement via an Optical Distribution Network (ODN) using an optical power splitter, which resides close to the user premises. Since the physical medium, of one or more communication links, is shared, the ONT's are scheduled by the OLT to transmit in the upstream direction in a Time Division Multiple Access (TDMA) manner.
In order to achieve high upstream bandwidth utilization the upstream scheduling must provide dynamic bandwidth allocation (DBA), which allows for bandwidth resource between lightly loaded and heavily loaded ONT's to be shared.
The Gigabit Passive Optical Networking (GPON) standard ITU-T G.984.x, introduces the concept of a Transmission Container (T-CONT). A T-CONT may be viewed as an upstream queue for a particular type of traffic (for example, video, voice and data). Each ONU typically holds several T-CONT's. The bandwidth assignment in the scheduling is done purely on a per T-CONT basis. Each T-CONT in the PON system is identified by a so-called Alloc ID. The OLT grants bandwidth to ONT's via a bandwidth map (BWmap) which comprises control signals sent in a downstream direction.
A Service Layer Agreement (SLA) associates each Alloc ID with respective bandwidth requirements to allow each Alloc-ID to be suitably serviced with bandwidth. The bandwidth requirements for one Alloc ID are described in terms of multiple bandwidth allocation classes. Each class has an associated bandwidth value, and together the values provide an overall bandwidth value for servicing each Alloc-ID. These limits define how a queue should be serviced with bandwidth. For example fixed bandwidth, assured bandwidth, non-assured bandwidth and best-effort bandwidth classes could be included in the SLA. Hence, a particular Alloc ID can be configured to obtain a certain amount of fixed bandwidth, up to a certain amount of assured bandwidth, up to a certain amount of non-assured bandwidth and up to a certain amount of best-effort bandwidth.
In order to be able to assign bandwidth to the T-CONT's according to need, the OLT may either utilize traffic monitoring or a messaging mechanism that has been introduced in the GPON protocol where status reports (containing queue occupancy) are transmitted to the OLT upon request. The OLT must, in addition to assigning bandwidth according to need, also enforce bandwidth guarantees, bandwidth capping and prioritization policies regarding traffic from different T-CONT's. The OLT is required to continually re-calculate how bandwidth is shared since the extent of queued traffic in each T-CONT varies over time.
We have realised that overly blunt, or rigid, DBA parameters result in certain services being given stricter service requirements than are actually required. This can result in sub-optimal bandwidth utilisation. We seek to provide an improved method of bandwidth allocation.